OMAP: DSS2: OMAPFB: fix crash when panel driver was not loaded

If the panel's probe had failed, omapfb would still go on, eventually
crashing.

A better fix would be to handle each display properly, and leaving just
the failed display out. But that is a bigger change.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>